1. Field of the Invention
The present invention relates to an image forming apparatus applied to a copier and printer. More particularly, the present invention relates to an image forming apparatus in which a toner image is fixed on a transfer sheet while a mold releasing. agent is being given onto the transfer sheet to prevent the occurrence of toner offset in the process of fixation, and further the image formation is automatically conducted on both the front and the reverse surface of the transfer. sheet while the transfer sheet is fed again to the transfer sheet holding rotary body, without reversing and temporarily accommodating the transfer sheet, on the first surface of which the fixation has been already completed.
2. Description of the Related Art
An example of this type full color image forming apparatus is shown in FIG. 5. This type full color image forming apparatus includes: a plurality of photoreceptor drums 100 on which toner images are formed by means of electrophotography while they are rotated in the direction of an arrow in the drawing; and a transfer belt 102 arranged at a transfer position of each photoreceptor drum 100 in such a manner that the transfer belt 102 comes into contact with each photoreceptor drum 100, wherein the transfer belt 102 is capable of simultaneously conveying a plurality of transfer sheets 101 onto which toner images have been transferred. In this image forming apparatus, toner images on the photoreceptor drums 100 are successively transferred onto the transfer sheets 101 fed onto and held by the transfer belt 102. After the completion of transfer, the transfer sheets 101 are made to pass in a fixing unit 103 in which the toner images are fixed while an oil-like mold releasing agent is given onto the transfer sheets. In this way, a color image can be formed.
In this type full color image forming apparatus, there is provided an automatic two-sided image forming means having "a trayless mechanism", which will be described below. In the process of two-sided image formation, the transfer sheet 101, onto the first surface of which the fixation has been completed, is made to pass in the sheet conveyance passage 104 shown by a dotted line in the drawing, so that the transfer sheet 101 is reversed. Then the reversed transfer sheet is fed again to the transfer belt 102 and held in a divided holding region on the transfer belt which is previously set in accordance with the length of the transfer sheet 101 in the conveyance direction before the start of image formation. While the transfer sheet is held in this divided holding region on the transfer belt, the image formation is conducted on the second surface.
In this connection, in the image forming apparatus including the automatic two-sided image forming means provided with the above trayless mechanism, the following problems may be encountered. Especially when the image formation is continuously conducted on both sides of a plurality of transfer sheets, a mold releasing agent coated on the first surface of the transfer sheet 101 in the process of image fixation on the first surface adheres onto the transfer belt 102 which reverses and holds the transfer sheet 101 in the process of image formation on the second surface. Further, the mold releasing agent which has adhered onto the transfer belt 102 moves to the photoreceptor drum 100 which comes into contact with the transfer belt 102. Therefore, the mold releasing agent which has adhered onto the photoreceptor drum 100 could be a cause of stripe-shaped oil stains that appear on a toner image.
Referring to an example in which images are formed on both sides of 7 transfer sheets of size A3 which are held and conveyed in a longitudinal direction on the transfer belt, adhesion of a mold releasing agent onto the photoreceptor drum caused in the process of automatic two-sided image formation will be explained as follows. FIG. 6 is a view showing a state of feeding in which the transfer sheets 101 are fed onto the transfer belt 102 in the process of automatic two-sided image formation. The transfer belt 102 shown in FIG. 6 is an endless belt, the entire circumference of which is developed into a sheet-shape for the convenience of explanation. FIG. 6 shows a state of the transfer belt in every revolution. In the example shown in the drawing, the entire length of the transfer belt 102 is divided into three equal portions, and the thus equally divided portions are defined as panels 1, 2 and 3. In this way, the divided holding region can be set in which each transfer sheet is held. In the drawing, "S" represents a transfer sheet fed and held for the formation of the first surface image, and "D" represents a transfer sheet fed and held for the formation of the second surface image. Also, in the drawing, "Skip" represents a panel for skipping on which no transfer sheet is held.
In the drawing, images are formed on both sides of transfer sheets as follows. While the transfer sheets (S-1) to (S-7) on which the first surface images are to be formed are made to skip by one panel, they are fed and held on the transfer belt 102, so that the first surface image formation is conducted. At the same time, the transfer sheets onto which the first surface images have been formed are successively fed again onto the transfer belt 102 via the trayless mechanism. In the second revolution of the transfer belt 102, one panel is made to skip on the panel 3. According to the above pattern, the transfer sheets are held on the transfer belt 102, and the second surface image formation is conducted. In this example, in the section from the panel 3 of the second revolution of the transfer belt to the panel 3 of the fifth revolution of the transfer belt, the first surface image formation and the second surface image formation are alternatively conducted.
However, on the transfer belt 102 onto which all transfer sheets for the formation of the first surface image have been fed, "empty panels" 105 are formed in which the transfer sheets are not held, that is, the panel 3 in the fifth revolution and the panel 2 in the sixth revolution are formed to be empty. In this two-sided image forming process, the transfer sheets finally held in "the empty panels" are the transfer sheets D-4 and D-5 on which the first surface image formation has been completed. Accordingly, the mold releasing agent that has been coated onto the first surface of the transfer sheet is made to adhere onto "the empty panel" 105. After such a transfer sheet has been peeled off from the transfer belt 102, "the empty panel" on the transfer belt 102 comes into contact with the photoreceptor drum 100. In the manner described above, the mold releasing agent coated on the transfer sheet 101 in the process of fixation adheres onto the photoreceptor drum 100 via the transfer belt 102.
In this connection, in the automatic two-sided image forming means having the trayless mechanism, a predetermined period of time is required for the transfer sheet, on the first surface of which an image has been formed, to arrive at a feed position on the transfer belt 102 via the trayless mechanism. Therefore, the panel on which the first transfer sheet after the completion of the first surface image formation is held after it has been fed again to the transfer belt 102, is actually limited to a panel which can hold the first transfer sheet conveyed again in a predetermined period of time in timed relation. In this example, the panel in which the first transfer sheet after the completion of the first surface image formation is held after it has been fed again to the transfer belt 102, is actually limited to the panel 3 in the second revolution.
Conventionally, there is disclosed a technique by which the problem of adhesion of a mold releasing agent to the photoreceptor drum can be solved in the case of two-sided image formation conducted on a plurality of continuous transfer sheets by an apparatus having a stack tray unit. This technique is disclosed in Japanese Unexamined Patent Publication Nos. 7-49599 and 7-181838. The apparatus having the stack tray unit is an automatic two-sided image forming means including an intermediate tray exclusively used for the two-sided image formation in which all transfer sheets are reversed and temporarily accommodated after the completion of fixation of the first surface image, and the transfer sheets accommodated in the intermediate tray exclusively used for the two-sided image formation are fed again onto the transfer belt in the process of the second surface image formation.
The above technique is described as follows. In the image forming apparatus, there is provided a transfer sheet carrier (transfer drum), arranged coming into contact with the photoreceptor drum, on which a plurality of transfer sheets are held so that a toner image on the photoreceptor drum can be transferred onto each transfer sheet. Transfer sheets, the number of which corresponds to a surplus obtained when the number of transfer sheets of continuous image formation is divided by the number of transfer sheets which the transfer drum can hold simultaneously, are first held on the transfer drum so as conduct image formation. That is, the second surface image formation is conducted on the transfer sheets accommodated in the intermediate tray after the completion of the first surface image formation. Specifically, the image formation is conducted as follows. When images are formed on both sides of 5 transfer sheets by a transfer drum capable of simultaneously holding 2 transfer sheets, the second surface image formation is conventionally conducted in the sequence of 2 sheets-2 sheets-1 sheet, however, in the above image forming apparatus, the second surface image formation is conducted in the sequence of 1 sheet-2 sheets-2 sheets while the transfer sheets are held on the transfer drum.
However, the aforementioned technique is an effective solution means only for an image forming apparatus having the stack tray unit by which images are formed on both sides of transfer sheets. That is, the aforementioned technique can not be applied to an image forming apparatus in which images are formed on both sides of transfer sheets by the trayless mechanism. The aforementioned technique is not an effective solution means.